How does Java run?

Java is a programming language that is cross-platform, its code runs through the Java Virtual Machine (JVM). Java code is compiled into bytecode files (.class files) first, and then interpreted and executed by the JVM.

Specifically, the process of running Java code is as follows:

1. Write Java code: Use the Java programming language to write source code and save it as a file with a .java extension.
2. Compile Java code: Use the Java compiler (javac) to compile source code into bytecode files (.class files). Bytecode is an intermediate code that is platform-independent.
3. Load bytecode file: JVM loads the bytecode file into memory.
4. Bytecode verification: The JVM verifies the loaded bytecode file to ensure its security and correctness.
5. Interpretation: The JVM interprets the bytecode file into machine code and executes it line by line. Interpretation execution means that the JVM will execute each bytecode instruction line by line, translating the instruction into machine code and performing the corresponding operation.
6. Just-In-Time Compilation (JIT): During runtime, the JVM will compile frequently executed bytecode into native machine code to improve execution efficiency.

Java achieves cross-platform capability by allowing developers to write code once and run it anywhere. Different operating systems just need to install the corresponding Java Virtual Machine (JVM) to be able to run Java programs.